1. Field of the Invention
The present invention relates to a process and an apparatus for the production of a ferromagnetic metal powder. More particularly, the present invention relates to a process for the continuous production of a ferromagnetic powder having excellent properties as a magnetic material for use in a magnetic recording medium, and an apparatus for use in the practice of the process.
2. Description of the Prior Art
Hitherto, .gamma.-Fe.sub.2 O.sub.3, Co containing .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co containing Fe.sub.3 O.sub.4, CrO.sub.2 and the like have been used as a ferromagnetic powder for a magnetic recording medium. These ferromagnetic powders, however, have not been suitable for use in magnetic recording wherein a signal of a short recording wave length (below about 10.mu.) is used, i.e., the so-called high density recording, because the magnetic properties such as the coercive force (Hc), the maximum residual magnetic flux density (Br) and the like are insufficient. Recently, in order to solve the above described drawbacks of the hitherto used ferromagnetic powders, a ferromagnetic powder is being extensively developed which has properties suitable for high density recording. Thus, various ferromagnetic powders of metals or metal alloys have been investigated and proposed. In general, as ferromagnetic metals, iron, cobalt and nickel are mainly used and, if desired, chromium, manganese, rare earth elements, zinc or the like is added.
The following processes for the production of the ferromagnetic substances are known.
(1) A method comprising decomposing an organic acid salt of a ferromagnetic metal by heating, and reducing in a reducing atmosphere.
This method is described in, for example, Japanese Patent Publication Nos. 11412/1961, 22230/1961, 8027/1965, 14818/1966, 22394/1968, 38417/1972, and The Record of Electrical and Communication Engineering Conversazione Tohoku University, Vol. 33, No. 2, Page 57 (1964).
(2) A method comprising reducing a needle-like oxyhydroxide or other metal containing needle-like oxyhydroxides, or a needle-like iron oxide as produced from these needle-like oxyhydroxides.
This method is described in, for example, Japanese Patent Publication Nos. 3862/1970, 20939/1964, 39477/1972, German Patent Laid Open (OLS) No. 2,130,921, British Pat. No. 1,192,167, and U.S. Pat. No. 3,681,018.
(3) A method comprising vaporizing a ferromagnetic metal in an inert gas.
This method is described in, for example, Japanese Patent Publication No. 27718/1972, and Oyo Butsuri (Applied Physics), Vol. 40, No. 1, Page 110 (1971).
(4) A method comprising decomposing a metal carbonyl compound.
This method is described in, for example, U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007, and 3,228,882.
(5) A method comprising electro-depositing a ferromagnetic iron using a mercury cathode and separating the mercury.
This method is described in, for example, Japanese Patent Publication Nos. 15525/1964, 8123/1965 and U.S. Pat. No. 3,156,650.
(6) A method comprising reducing a ferromagnetic metal salt in a solution thereof.
This method is described in, for example, Japanese Patent Publication Nos. 20520/1963, 26555/1963, 20116/1968, 41718/1972, U.S. Pat. Nos. 3,663,318, 3,661,556, German Patent Laid Open (OLS) Nos. 2,132,430, 2,326,258 and 2,326,261, and U.S. Pat. Nos. 3,206,338, 3,494,760, 3,567,525, 3,535,104, 3,607,218, 3,756,866, 3,669,643, 3,663,318, 3,772,867 and 3,790,407.
The present invention is concerned with an improvement in the Method (6) above wherein a ferromagnetic metal salt is reduced in a solution thereof, and particularly, the present invention is concerned with the use of a borohydride compound or derivative thereof as a reducing agent.
It is known that the oxidation-reduction reaction using borohydride or derivative thereof as a reducing agent is generally vigorous. For instance, when sodium borohydride is used as a reducing agent, a large volume of hydrogen gas is liberated simultaneously with the beginning of reaction, and thus the reaction solution is bubbled, the ferromagnetic powder produced rises to the upper portion of the reaction vessel together with the bubbles, the reaction volume is materially increased and the reaction solution often overflows. The phenomenon that the reaction volume is increased, often tends to take place particularly markedly where an additive, e.g., a surface active agent, an organic solvent, and the like is added to the reaction solution in order to improve the magnetic properties or to increase the stability of the magnetic substance to oxidation. As stated above, since the oxidation-reduction reaction is vigorous and the ferromagnetic powder produced is highly active, many difficulties have been encountered in the procedure. Furthermore, the reaction has been required to be carried out in a magnetic field in order to improve the magnetic properties of the ferromagnetic powder produced. In order to effect the reaction industrially while fulfilling the above requirements, various methods have been investigated from various standpoints, but these methods are not satisfactory with respect to economy, reaction stability and so on. Machese et al disclose a process in U.S. Pat. No. 3,684,484 wherein individual reactant fluids enter a mixing zone in a controlled laminar flow, the reactant fluids are merged in the mixing zone where finely divided metallic particles are produced by chemical oxidation-reduction. In one application Machese et al disclose the use of a magnetic field located downstream from the point at which the reactant streams merge. Little discloses a procedure for producing ferromagnetic alloy particles in U.S. Pat. No. 3,672,867. In accordance with this procedure, two reacting streams impinge forcibly and mix. Reaction takes place upon mixing. Optionally a solenoid or gap of a permanent magnet may be located at and immediately after the point of impingement, however, this latter procedure has been of questionable value.